In order to enable a transmission of phase and amplitude information of a signal via the radio interface, the signal first has to be converted into a radio-frequency signal comprising the original phase and amplitude information. For such a conversion, EER transmitters offer a better efficiency than traditional IQ-modulator architectures, which makes EER transmitters of particular interest for mobile devices. The better efficiency is achieved especially for linearly modulated signals for which the peak-to-average ratio (PAR) can be quite high.
In an EER transmitter, first the envelope of the signal that is to be transmitted is eliminated. The resulting constant-amplitude phase modulated signal can then be amplified efficiently using very non-linear power amplifiers, such as class-E switching mode power amplifiers. An amplitude modulation of the power amplifier can be used to restore the envelope and thus the amplitude information of the original signal. In practice, this should take place by controlling the supply voltage of the power amplifier in order to preserve its good efficiency. Linear power amplifiers often cannot be modulated in this way, since small changes in their supply voltage do not affect the output signal amplitude.
In most applications, it is required in addition that the average power of the signals output by the transmitter be controlled.
Due to the use of very non-linear power amplifiers, the conventional approach for realizing a power control cannot be used for EER transmitters. In a conventional power control, the power of signals which are input to a power amplifier is adjusted, e.g. by means of a variable gain amplifier, and the adjustment appears correspondingly at the output of the power amplifier. The output power of very non-linear power amplifiers, as employed in EER transmitters, however, is not affected by a change of the input power.
FIG. 1 is a block diagram illustrating schematically a known approach for controlling the output power in an EER transmitter.
The block diagram comprises a modulator 101, which is connected to a highly efficient but very non-linear power amplifier 104. In addition, a battery 111 and a control signal generator 112 are connected to a highly efficient switching mode power supply (SMPS) 113. Instead of the SMPS 113, also a less efficient linear regulator could be used. The output of the SMPS 113 is connected to a supply voltage input of the power amplifier 104.
The modulator 101 provides a radio-frequency signal, which constitutes the phase-modulated part of the desired output signal, for amplification by the power amplifier 104. The control signal generator 112 provides the SMPS 113 at the same time with a control signal which represents a combination of desired amplitude modulation of the output signal and the currently desired power level of the output signal. The SMPS 113 regulates a voltage received from the battery 111 according to the received control signal and provides the resulting voltage to the supply voltage input of the power amplifier 104. The signal provided by the modulator 101 is then amplified by the power amplifier 104 with an amplification factor depending on the current voltage supply. The output of the power amplifier 104 constitutes at the same time the output ‘Out’ of the EER transmitter.
Thus, the required dynamic range for the amplification has to cover both the desired amplitude variation and the average power level variation. The dynamic range that can be achieved by the SMPS and by the power amplifier, however, is restricted by some lower limit. The lower limit for the power amplifier results from a leakage of the input signal through the power amplifier transistor due to its parasitic capacitances.
In U.S. Pat. No. 6,323,731, it is proposed to employ a dynamic bias control for the power amplifier, in order to widen the output power range compared to the approach of FIG. 1. Nevertheless, the achieved range is still limited.